Inspecting apparatus



28, 1953 G. w. PENNEY ET AL INSPECTING APPARATUS Filed April 4, 1947 INVENTOR 5 Gay/0rd M Penney and WITNESSES:

flora/a H Thomas,

ATTORNEY Patented July 28, 1953 STAT.ES

PATENT OFFICE- IN SPECTING APPARATUS Application April 4, 1947, Serial No."Z39,, 16

4Claims.

Thisinvention relates'to apparatus for inspecting or examining materials and, more particularly, to apparatus for examining materials'by the X-ray absorptionmethod. The invention has i applicability to the measurement of the thickness of steel strip as-it passes through a rolling mill.

Arrangements fo examining materials by the X-ray absorption method make use of the wellknown .fact that the absorption of monochromatic X-rays by a homogeneous material varies with the thickness of the-material. 'The amount of absorption by a material of a given thickness depends upon the particular material and the wave. length of the X-rays.

In the past, diiferences'in absorption of X-rays ior'examination purposes have been determined by the use of photographic film exposed to the X-rays after passage, thereof through the material Such'an arrangement requiresa considerable time to obtain the result desired, since a relatively long exposure must be made and the film processed and inspected.

A number of possible uses of the X-ray absorption method of examining material become impractical when a long time is required to obtain the results. For example, in connection with the. rolling operationsin a steelmilL-it is desirable to measure the thickness. of a'substantiall-y homogeneous steel strip Without physical contact with the strip. The thickness of the; strip may, of course, be measured by the X-ray absorption method. However; the strip is moving at a very high speed and an indication of the thickness is desired to permit correction and control of the rolling apparatus; To be useful, such as indication must, therefore; be obtainable very quickly and accurately.

In an attempt to" avoid any undue time delay, more recent arrangements make, use of a photo- 'tube exposed to light from a fluorescent screen Another object of our invention is to provide novel apparatus for measuring the thickness of a homogeneous material.

A further object of our invention is to provide novel apparatus for measuring the thickness of a homogeneous material in which an indication of the thickness relative to a standard is obtained quickly and accurately.

In accordance'with our invention, X-rays after passage through the material to be examined, are received by an ionization chamber. The chamber may be of the multiplate type and the ion current'produced' therein is a measureoi the X-ray absorption of the material. A special amplifier arrangement is used in conjunction with the ionization chamber to amplify the ion current and supply it to a curro nt'measuring, device. The multiplate ionization chamber and special amplifier arrangement enable a quick response to ion current changes to be indicated on the measuring device.

The features of our invention which are considered novel are set forthwith more particularity in the accompanyingclai-ms. The invention itself, however, together with additional objects and advantages thereof, may be better understood from the following description of specific apparatus when read in connection with the accompanying drawing, in. which:

Figure l is a schematic circuit diagram of a preferred embodiment of our'invention employed as a thickness gage; and

Fig. 2 is a schematic diagram of a modification of the apparatus of Fig. 1.

As shown in Fig. 1, a source of X-r'ays such as an X-ray tube 3 isjprovided and is, energized from an alternating voltage source 5 through a transformer l and a full wavev rectifier 9. X- rays from the X-ray tube 3 are projected toward a pair of metal strips H and 13. One of these strips l l is considered as the test strip of unknown thickness and may be strip moving between rolls of a rolling mill. The other strip [3 is of the same materialand is considered as the standard strip of known thickness. To avoid complicating the drawing, the apparatus for supporting the strips and the rolls are not indicated.

The strip H will, however, often be at a high temperature.

The test strip H and the standard strip l3 are positioned side by-side in a plane substantially perpendicular to the beamof X-raysirom the X-ray tube 3. The tube '3' is preferably positioned with respect to the strips so that substantially the same quantity of X-rays is incident on each of the strips.

A pair of ionization chambers i and H are provided, one corresponding to each of the two strips I! and I3. The chamber l5 corresponding to the test strip H is positioned on the opposite side thereof from the X-ray tube 3 and is arranged to receive X-rays from the tube after passage thereof through the test strip. Similarly, the chamber H corresponding to the standard strip I3 is positioned on the opposite side thereof from the X-ray tube 3 and receives X-rays after passage thereof through the standard strip. The ionization chambers l5 and H are enclosed in a case [9 of a material having a high X-ray absorption such as lead. A pair of openings 2! and 23 are provided in the case [9 to permit the entry of the X-rays. Over each of these openings is mounted a thin dust cover of a material having a very low X-ray absorption such as aluminum. The covers for both chambers l5 and I! are of the same material and thickness.

As Y-rays from the X-ray tube 3 pass through the test strip l l or the standard strip l3, some of the X-rays are absorbed in accordance with the thickness of the strip. Those X-rays which are not absorbed are received by the corresponding ionization chamber and produce ions therein so that the ion current in an ionization chamber is a measure of the thickness of the strip through which the corresponding X-rays pass. It is then necessary to amplify and measure the ion current of each chamber to obtain an indication of the thickness of the corresponding strip.

Ionization measurements in the past have been noted for the slow response obtainable. This slow response was due primarily to two factors, first, the time required to sweep out the ions and, second, the capacity of the collecting electrodes. In accordance with our invention, we propose to overcome the slow response in ionization measurement by employing a multiplate ionization chamber and a direct current amplifier having a negative feedback circuit between the output and input circuits of the amplifier causing the overall voltage amplification to be of the order of unity.

By using a multiplate ionization chamber, the distance which an ion must travel to reach a plate is reduced so that the ion transit time becomes negligible. In addition, the space charge effects are reduced in a multiplate ionization chamber so that charges resulting from ionization can reach the collecting surface quickly. However, the multiplate ionization chamber has a large capacity which tends to result in a slow response of the apparatus. The changes in ion current produced in the ionization chamber are rather small and, consequently, in an amplifier therefor, a large amplification and a coupling or input resistance of a high value are required. Such a large input resistance in conjunction with the large capacity of the chamber introduces a large time constant in the input circuit of the amplifier. Th ion current produced in the ionization chamber flows through the high input resistance to produce a voltage thereacross resulting in a voltage change across the ionization chamber and the capacity of the chamber is so large that this change cannot take place rapidly.

In using the expression multiplate ionization chamber in the claim, I mean a chamber havin the following attributes:

(1) A plurality of plates as distinct from two plates (one positive and one negative);

(2) Plates spaced so closely that the ion transit time between plates is negligible;

(3) Relatively low space charge eilect;

(4) Relatively high capacity.

In accordance with our invention, a negative feedback circuit is provided between the output and input circuits of the direct current amplifier which, in effect, causes the output voltage to be in series with and in opposition to the voltage produced across the input resistance by the fiow of ion current therethrough. By making the gain of the amplifier rather high, the output voltage may be made almost equal to that developed across the input resistance. As a result, the actual change in voltage across the ionization chamber can be made extremely small and is approximately l/A of the amount of change which would be made without the use of the feedback circuit, where A is the gain of the direct current amplifier without the feedback. It is then apparent that with a high gain, the amount of voltage change across the ionization chamber is but an extremely small fraction of the change necessary with a conventional amplifier arrangement. Obviously, this extremely small change may take place quite rapidly so that the long time constant is eliminated.

With such an arrangement the voltage gain of the amplifier is of the order of unity. However, the current gain is approximately the ratio of the input resistance to the output resistance and may be of the order of twenty thousand to twenty million in a practical design.

For the arrangement shown in Figur 1, a direct current amplifier is provided which has two channels, one for the chamber l5 associated with the test strip l l and one for the chamber l1 associated with the standard strip l3. The first or input tube 21 of the test channel is preferably a pentode having its anode 29 connected through a resistor 3| and a ortion of a potentiometer 33 to the adjustable tap 35 thereon which is connected to the positive terminal of a direct current voltage source 31. The negative terminal of the voltage source 31 is connected to the positive terminal of a second direct current voltage source 33, the negative terminal of which, in turn, is connected to the positive terminal of a third direct current voltage source 4| having a connection from its negative terminal to the ground.

The cathode 43 of the input tube 21 is connected to the junction point between the second and third voltage sources 39 and 4|. The suppressor grid 41 of the input tube 21 is connected to th cathode 43 and the shield grid 49 is connected to the junction point 5i between the first and second voltage sources 31 and 33. The control grid 53 of the input tube 21 is connected to one set of plates 55 of the ionization chamber l5 while the other set of plates 51. is connected to the positive terminal of the first voltage source 31.

The input tube 21 is supplemented by a second amplifier tube 59 which is enclosed in the same envelope with a similar and corresponding tube 6| employed in the other channel. Additional amplifier tubes may, of course, be employed as desired, but the total number of tubes in the channel should be an even number.

The anode 63 of the second amplifier tube 59 is connected to the positive terminal of the first voltage source 31. The cathode 65 of the second amplifier tube is connected through a portion of a. resistor 61 to the grounded intermediate tap 69 thereon. The control electrode II of the second rinputtubefllof: the second channel-is connected :fthtoughna resistor 18 I a'n'd a -portion.-:of-:.the1 potentiometer 33 to the adjustable.ztapzc35'rtheteon; whichsisconnectedvto therp'ositive terminal of the firstcvoltagersourceil. aThe cathode 83 of the input tube 11 is connected to the junction point i 45 between second andthird-voltage sources 39 and 4| and thejsuppreswr gridpz85gis connected to thecathode 83. The shieldgrid 8! of the input tube 7,1L'I1is connected to the .;iunction. point .5l

between the first and second voltage sourcesfs' if and 39.,1'The control grid'89 of the input/tube]? isconnected 'to one set ofplatesjfil of theioniza- .tio'n Chamber 11 whilelthe' other set,,of plates '93I;is..conne;cted' to the positive terminaluofgthe voltage source3l. '.,Tne,,ano de' 95 of the second amplifiertubefil .offthejstandard channel is connected tothepo'si tire terminal of thefirst' voltage \SOUICE'LSJ .wh'ile the cathode 91' is connected'through a portionof the resistor BIrto the grounded intermediate tap 69 thereon. The controlgrid 8-9 of the.- sccon d almpli fierc-tube 6| is connected..-tq the anode I9 flofstherinput tube 11 of the second channel. 1

.It is tG-Jb noted that-the cathodes o flall four .tuheSI-ZI SQ, BI and "I"! of the,amplifier =are,.in directly heated but the heating circuits are omitted from therirawing-forpurposes of clarity. d'The-econtrolselectrodecfiflcofi the.-;input-..-tube" I1 is connectedithroughcan input resistonwl fll, 118V.-

ing a high resistance, and a seocndblasingvuoltage source I93 to the cathode 91 of the second. amplifying tube 6|. Thus a negative feedback circuit is also provided for the second channel. It is also evident that the ion currents in both ionization chambers are amplified by substan input tubes 2! and TI of both channels of the amplifier may be RCA-6SJ7 tubes, and the twin tube employed as the second amplifier tubes 59 and GI in both channels may be an RCA 6SN7 tube. The resistors described may have the following values:

Cir

T e aw eee. s urces .ma have the iollowing values;

" veltageiscurcelhld J'The' icnizati nachambers-iwere eithermultiiilate styapeihaying' ammum- .capacitance'of. theicrder 10f. aoliimicrofan'ad zalldi'. thevmeas-uring adeviceawas zaiO-jd-microampmeter.

'Aalthcugh-twe::haveiillustrated the X-ray'ctube :asazpositioned so that substantially vthe same QIlEIaMitYQOfLX-TEIYS:iSnTPIOjECtGdLOII to eachistrip, itrmaysbe ipos'itione'd .otherwise. :Thexobj ec't, 10f *ccnrse ,ci's'*- to acquire .a' balance meter .:at tanzeromeadingwhen thehtwo strips armofz the .sametthiclmess. :Byrhazving. far-single- -X.-*rayz-tube supply. Kerays for; both 1 stripsgvanyevariation sin Xecay.intensityzbecauseof variations insource auoitage. is balanced .out.;

ifl-mtlie-sarra-ngement shownijn Fig; 1, twoioniizaticncchambers -.I 5.;and :l I @are employed; tones-ref which acts .as. aimonitor eto. indicatethe absorption ofnXerays bygthe strip' '13 of standard-thick?- ness wihioh 'is; compared with -:that. of test: :strip I e Suchzan;.a'rrangementaminimizesanyr-efiects. rofffiuctuations in rthevxeray .outputiof ,the;X.-ray tube 33 :such as-rmay :be caused.- by :Xeray aatube :supplye'uoltage changes. .zI-Icwever; .inzztheq-event .suckmmonitcring action :is. not. snecessarypan 28.1- rangementpmay ibe temployed. in which ;:but is mingle.-ionizatiomchamben'is :usedito, measure the .-Xemt-y rabsorption ;@by-,.the :material :to mbeaexanii'nedeor stestedr zfiuch can .aarrangementcimay iemployz mcircuitiaszshowmin:Fig: 2. i

The circuit of Figgizccerisespondsuto:ia =:.single channeli:tifvtheaamplifier:of Fig.1 1:;andithes same reference: numbers Ia-reapplied to 1 corresponding elements. It is apparent that the only departure of the circuit of Fig. 2 from that of a single channel of the circuit of Fig. 1 is in the connection of the current measuring device I05 which is necessarily placed directly across the output resistor 61a.

In this arrangement, the current measuring device is calibrated to read the absorption directly. Thus, any change in the reading of the measuring device I05 would indicate a change in the absorption of X-rays by the strip being tested.

Although we have shown and described specific embodiments of our invention as applied to measuring th thickness of a material, we are aware that many other applications of our invention as well as other embodiments may be made by those skilled in the art without departing from the spirit of the invention. We do not intend, therefore, to have to limit our invention to the particular embodiment or arrangement shown and described herein. y

We claim as our invention:

1. Apparatus for examining an object comprising means on on side of said object for projecting a beam of X-rays toward said object, an ionization chamber on the other side of said object for receiving X-rays of said beam after passage thereof through said object whereby an ion current is produced in said chamber, a direct cur-- rent amplifier connected to said chamber to amplify said ion current, a negative feedback circuit between the output and input circuits of said amplifier including components having dimensions causing the overall voltage amplification to be of the order of unity, and a current measuring device in said output circuit.

2. Apparatus for examining an article comprising means on one side of said article for projecting a beam of X-rays toward said article, a multiplate ionization chamber on the other side of said article for receiving X-rays of said beam after passage thereof through said article whereby an ion current is produced in said chamber, a direct current amplifier coupled to said chamber with an input resistor through which said ion current flows, circuit means connecting said input resistor to the output circuit of said amplifier causing the input resistor voltage produced by said ion current to be opposed by the output voltage of said amplifier, said circuit means including components dimensioned so that the gain of said amplifier causing said output voltage to equal substantially said input resistor voltage, and a current measuring device in said output circuit.

3. Apparatus for comparing two articles, comprising a source of X-rays adapted to project X-rays on each of said articles, an ionization chamber associated with each of said articles for receiving X-rays from said source after passage thereof through the corresponding article, whereby an ion current is produced in each chamber, a direct current amplifier having two amplification channels corresponding to said chambers, each channel being connected to the corresponding chamber to amplify the ion current thereof, a negative feedback circuit between the output and input circuits of each channel having components having dimensions causing the overall voltage amplification of each channel to be of the order of unity, and a current measuring device connected in the output circuit of both channels to indicate the difierence between the output voltages thereof.

4. Apparatus for comparing two articles, comprising a source of X-rays adapted to project X-rays on each of said articles, a multiplate ionization chamber associated with each 0! said articles for receiving X-rays from said source after passage thereof through the corresponding article, whereby an ion current is produced in each chamber, a direct current amplifier having two amplification channels corresponding to said chambers, each channel being coupled to the corresponding chamber with an input resistor through which the ion current of that chamber flows, circuit means connecting the input resistor to the output circuit of each channel causing the input resistor voltage produced by the corresponding ion current to be opposed by the corresponding output voltage, said circuit means having components such that the gain of each channel causing the output voltage thereof to equal substantially the corresponding input resistor voltage, and a current measuring device connected in the output circuit of both channels to indicate the difference between the output voltages thereof.

' GAYLORD W. PENNEY.

HAROLD A. THOMAS.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 2,094,318 Failla Sept. 28, 1937 2,097,760 Failla Nov. 2, 1937 2,264,725 Shoupp et a1 Dec. 2, 1941 2,346,486 Hare Apr. 11, 1944 2,349,429 Herzog et al May 23, 1944 2,365,763 Kalb et a1 Dec. 26, 1944 2,458,099 Roop Jan. 4, 1949 2,465,938 Shonka Mar. 29, 1949 2,536,617 Weller Jan. 2, 1951 2,537,914 Roop Jan. 9, 1951 FOREIGN PATENTS Number Country Date 293,240 Germany -July 27, 1916 554,001 France Feb. 21, 1923 567,280 Great Britain Feb. 6, 1945 601,545 France Dec. 1, 1925 OTHER REFERENCES Terman, Radio Engineers Handbook, McGraw- Hill Book Co., 1943, pp. 395-397 and 402. (Copy in Div. 54.) 

